William Dong, of Stratfield Road, left, in state Superior Court the day after his arrest last Dec. 3 on multiple weapon charges, shown with Assistant Public Defender Kevin Williams. On Tuesday, federal prosecutors filed a new charge against Dong of unlawfully transporting into the state an assault weapon he purchased in Pennsylvania.

William Dong, of Stratfield Road, left, in state Superior Court the day after his arrest last Dec. 3 on multiple weapon charges, shown with Assistant Public Defender Kevin Williams. On Tuesday, federal

William Dong, the Fairfield man who triggered a lockdown at the University of New Haven campus last month by bringing loaded weapons on campus, now faces a federal gun charge in addition to state charges filed against him.

According to a statement issued Tuesday by the office of Deirdre M. Daly, the U.S. attorney for Connecticut, the 23-year-old Dong has been charged with the federal violation of unlawfully transporting into the state an assault weapon purchased in Pennsylvania.

The complaint was unsealed Tuesday during Dong's appearance in New Haven federal court before U.S. Magistrate Judge Joan G. Margolis. He agreed to the entry of a federal order of detention and waived his right to a speedy indictment and a probable cause hearing.

Dong, who is being held in lieu of $500,000 bond, was charged Dec. 3. after his arrest on the West Haven campus with state complaints of illegal possession of an assault weapon, illegal transport of an assault weapon, breach of peace and illegal possession of a weapon in a motor vehicle.

His lawyer has said he plans to plead not guilty to the state charges when he next appears in Milford Superior Court.

The federal prosecutor said that Dong, who lived on Stratfield Road with his family, had traveled to Pennsylvania last September to buy a Bushmaster model XM-15-E2S, .223-caliber semi-automatic rifle and then brought the rifle back to Connecticut. The assault rifle is considered a prohibited assault weapon under state law.

When Dong was arrested Dec. 3 by West Haven police, they said he had two handguns in his possession and the Bushmaster rifle was in the car that he had parked nearby, according to Daly's statement. Dong told police that he purchased the rifle from someone in Pennsylvania through an advertisement he saw on the website, www.armslist.com.

"Although it is not unlawful under federal law for an individual, who is not a prohibited person, to possess this Bushmaster firearm," Daly said, "it is a federal violation for an individual to purchase this firearm outside of Connecticut and travel into the state with it, since it is a prohibited firearm under Connecticut state law."

The federal charge carries a maximum penalty of five years in prison and a $250,000 fine.

In addition to the loaded handguns that West Haven police said Dong had when he was taken into custody without incident, as well as the Bushmaster rifle that was found in his car, police said they found five magazines with 30 rounds of ammunition each next to the rifle, along with three magazines with a total of more than 50 rounds for the handguns.

Later in the day, when West Haven police executed a search warrant at Dong's Fairfield home, assisted by Fairfield police, they found his bedroom door padlocked.

After officers broke open the door, police sources said they found 2,700 rounds of ammunition and newspaper clippings about the mass shooting in an Aurora, Colo., movie theater where 12 people were killed and nearly 60 others injured in July 2012.